The present invention generally relates to improved methods and apparatuses for driving light emitting diodes (“LEDs”), such as those used in LED lamps and lighting fixtures, and improved methods and apparatus for driving that facilitate locating driving circuitry and the LEDs together on a single package.
Typically, the use of LEDs as an AC powered light source requires a driver to convert AC line power to a regulated DC power for maintaining constant light output. Such an LED driver is a self-contained power supply that has output matched to the electrical characteristics, such as forward voltage and driving current, of the LED. Usually, the driver is built on an individual PCB and connects to an LED PCB which contains the array of LEDs.
FIG. 1 shows typical circuitry of an AC-DC LED driver using the known flyback converter topology. The circuitry 10 includes an AC power source 12 with live (L) and neutral (N) terminals, and a rectifier circuit 14 that converts the AC, e.g., sinusoidal, input waveform to a DC waveform, e.g., a half wave rectified waveform. The circuit operates such that when the switch Q1 conducts, current is directly drawn from the rectified sinusoid. Energy is stored in the magnetizing inductance of the primary winding of transformer T1. The rectifying Zener diode D1 is reverse biased and the LED current is supplied by the secondary capacitor Cout. When Q1 turns off, diode D1 conducts and the stored energy is delivered to the secondary winding of transformer T1 and to the output. The controller chip U1, which can be, for example an iW3620 Digital PWM Constant Current Controller for AC/DC LED Driver, manufactured by iWatt, Inc., regulates the LED current by comparing the information about the secondary output voltage and LED current, which information is reflected via the auxiliary winding of transformer T1, to a constant reference and, based on the comparison, adjusting the duty cycle of switch Q1.
AC-DC drivers such as the one shown in FIG. 1 are complex and bulky circuits. With the goal of making a simpler circuit, LED manufacturers have developed an AC LED circuit, which is basically LED lighting circuitry that can operate on AC power without the need for the complex AC-DC driver of the type shown in FIG. 1. However, the AC line voltage is sinusoidal and the majority of low frequency AC LED systems operate in the rectified mode, which turns off the AC LED at a rate of double the line frequency.
FIG. 2 is a schematic of a conventional AC LED circuit. In such a circuit, the LEDs can be driven directly by the AC power source 102 without the use of a complex converter. In operation of the illustrated circuit, during the positive half cycle of the, e.g., sinusoidal, AC source V, LED string S2 is reverse biased and LED string S1 is conducting and emitting light. During the negative half cycle of AC source V, LED string S1 is reverse biased and LED string S2 is conducting and emitting light. The forward voltages of LED strings S1 and S2 are equal. The resistor R limits the current through the LED strings S1 and S2.
FIG. 3 shows the light output (i.e., luminous flux vs. time) of an AC LED circuit. It can be seen from the figure that the off-time in such a circuit is about 40%, due to the fact that the LED current of the AC LED circuit is discontinuous at the zero crossing of the AC voltage waveform. Since the rectified AC line voltage cycles from zero to peak level and back to zero, the LED string turns off whenever the line voltage level falls below the forward voltage of the LED string.